[1] The battle was fought in the autumn of 799 between the defending forces of Dalmatian Croatia under the leadership of Croatian duke Višeslav, and the invading Frankish army of the Carolingian Empire led by Eric of Friuli.

[5][2][7] The Frankish invasion of Croatia, the destruction of Tarsatica, the coronation of Charlemagne as Holy Roman Emperor, and negotiations from 802–815 between the Franks and Byzantines led to a stalemate.

[10] In return for the help of Charlemagne, Vojnomir was obliged to recognize Frankish sovereignty, convert to Christianity, and have his territory named Principality of Lower Pannonia.

[13] Dalmatia at that time included both Roman cities and a Slavic-Croatian hinterland that was loosely subject to the rule of the Byzantine Empire.

[13] In the treaty of 798, the Franks acknowledged Byzantine rights over the Slavs, but in the following years both Croatian Župans (dukes) and Roman communities recognized an opportunity to win full independence from both Imperial powers.

[13] As the eldest son of Gerold of Anglachgau and as a high ranking Frankish commander, Eric was titled from 789 to his death the Duke of Friuli (dux Foroiulensis).

[Note 2][1][12][14] In the autumn of 799, Eric marched from Istria along the seacoast of Liburnia towards the town of Trsat, which is today part of the city of Rijeka.

Led by Duke Višeslav, the inhabitants of Trsat threw spears, shot arrows, and hurled huge stones on the enemy, and managed to kill many of them.

[8][14][16] Aquileian Patriarch Saint Paulinus II cursed the land in which the hero was killed, and wrote Carmen de regula fidei, the rhythmus or elegy for his death.

[14] According to contemporary Frankish scholar and courtier Einhard, Eric was killed at Trsat (Tarsatch), a town on the coast of Liburnia, by the treachery of the inhabitants.

[14][19] On Christmas Day in 800, Pope Leo III crowned Charlemagne as Imperator Romanorum (Holy Roman Emperor) in Saint Peter's Basilica.